Published: September 09, 2011 r2011 American Chemical Society 4113 dx.doi.org/10.1021/nl2023788 | Nano Lett. 2011, 11, 4113–4117 LETTER pubs.acs.org/NanoLett Molecular Calipers Control Atomic Separation at a Metal Surface Lydie Leung, † Tingbin Lim, † John C. Polanyi,* ,† and Werner A. Hofer ‡ † Lash Miller Chemical Laboratories, Department of Chemistry and Institute of Optical Science, University of Toronto, 80 St. George Street, Ontario, M5S 3H6, Canada ‡ Surface Science Research Centre, The University of Liverpool, Liverpool, L69 3BX, U.K. b S Supporting Information S ingle molecules have been proposed as rulers for measuring molecular length, particularly for biomolecules. 1À3 If the ruler were actively engaged in controlling the molecular length of some other specie, it could be termed a “molecular caliper”. This designation has been used to describe the function of yeast enzymes (“Elop”) thought to control molecular chain-length in certain fatty acids. 4 The term “caliper” describes a measuring instrument, or, as here, a marking device “for scribing lines at specified distances”. 31 The calipers demonstrated here are com- prised of linear polymers of p-diiodobenzene, (pDIB)n, of varying length, physisorbed on cooled Cu(110). As a consequence of electron-induced reaction these chemically imprint their terminal I-atoms at controlled separations on the copper surface. The physisorbed monomer or linear polymer of variable length, there- fore, constitutes the molecular caliper. Structurally a caliper must not distort prior to reaction; functionally the chemisorbed reaction product formed at the surface must be localized in the vicinity of its former position in the caliper. Neither condition is assured since physisorbed reagent can isomerize, and reaction products migrate. These conditions for functioning of a molecular caliper are, however, shown to be met in the present instance. Structural rigidity of the reagent caliper was ensured by the stability of benzene rings π-bonded flat on metal surfaces at low temperatures. 5À9 The localized nature of the I-atom reaction at the metal surface, constitutes a significant finding, anticipated by Maksymovych and Yates who showed a preponderance of reten- tion of conformation in the electron-induced dissociation of CH 3 SSCH 3 to give CH 3 S pairs at Au(111). 10 Localized atomic reaction (LAR) has been reported extensively in the halogenation of silicon. 11,12 Localized reaction ensures that the separation of the I-atoms in the molecular caliper correlates with their subsequent separation at the surface. Earlier scanning tunneling microscopy (STM) studies of surface halogenation by dihalobenzenes gave the pattern of photoinduced 11 and thermal 13À15 reaction at a semiconductor surface, Si(111)-7 Â 7. The three principal findings were that the halogen atom recoiled along the CÀX bond direction, reaction was localized to the vicinity of the original position of the X-atom in the adsorbed reagent, and chemisorbed pairs of X were found at a separation exceeding by a few angstroms the previous separation in the physisorbed reagent. A simple model 16 gave values of 0.3À0.4 nm for the increase in pair separation from the adsorbate to the final chemisorbed state. The thermal dissocia- tion of pDIB on Cu(111) has been studied by McCarty and Weiss 17 who found pairs of I-atoms 0.4 nm apart, markedly less than the initial separation of 0.72 nm in the undissociated molecule. The I-atoms exhibited mobility on room temperature copper, which could therefore account for the observed short pair separation. The reaction dynamics reported here, involving an increase in I..I separation in reaction, resemble those reported above for dihalobenzenes on silicon. In the present instance, however, the reaction was electron-induced at a metallic (rather than a semiconductor) surface, and the terminal halogen atom separa- tion in the physisorbed reagent was systematically varied over the range 0.69À2.89 nm by collinear self-assembly. The end-to-end I..I separation in the reagent will be shown to correlate closely with the subsequent I-atom separation at the surface. The reagent acted, therefore, as a molecular caliper used to mark the surface with chemisorbed atoms at a set distance apart. Received: May 19, 2011 Revised: September 2, 2011 ABSTRACT: If a molecule controls the length of some other moiety, it can be termed a “molecular caliper”. Here we image individual molecular calipers of this type by scanning tunneling microscopy. These consist of linear polymers of p-diiodoben- zene, (pDIB)n, of varying length, 0.7À2.9 nm, physisorbed on Cu(110) at 4.6 K. Through electron-induced reaction these chemically imprint their terminal I-atoms on the copper, 0.7 nm further apart than their initial separations. The physisorbed monomer or polymer, therefore, constitutes a molecular-caliper with variable terminal I..I separation. The localized nature of the I-atom reaction at the copper surface relative to the parent molecule, constitutes a novel finding reported here. It ensures that the separation of the I-atoms in the physisorbed molecular caliper correlates with their subsequent separation when chemisorbed at the surface. KEYWORDS: STM, electron-induced reaction, halogenation, metal surface